The invention pertains to the timing of the intake and exhaust valves of internal combustion engines, and more particularly to varying the timing such that the valves remain open a longer portion of the engine's operating cycle at higher RPM than at lower RPM. The invention is directed particularly to, but not exclusively to, high performance engines having a modified camshaft.
When an engine is cammed for more power, its ability to breathe is effectively increased by opening the valves wider and holding them open longer. Keeping them open longer is known as increasing the duration of the cam. Theoretically, the exhaust valve of each cylinder opens when the piston reaches bottom dead center of the power stroke and remains open as the piston rises to top dead center on the exhaust stroke. At this point, the intake valve opens and the exhaust valve closes. This is, generally speaking, how the valves are timed on a stock engine.
In order to extend the total duration of the valves, the exhaust value must open before the piston reaches bottom dead center and close after the piston reaches top dead center, and the intake valve opens before the piston reaches top dead center and closes after reaching bottom dead center. This gets into a situation known as overlap: the intake valve opens before the exhaust valve is fully closed, producing a period in which both valves are open simultaneously.
At high RPM, despite the overlap, the increase in the ability of the engine to breath increases engine power. However, at low RPM, just the opposite is true, and the more radically varied the cam is, the greater is the overlap period, and the greater the power loss at low RPM. A car modified with a radical cam is hard to start, idles very roughly, and lacks low speed power.
This problem is addressed in U.S. Pat. No. 3,921,609, issued to the instant inventor's father on Nov. 25, 1975. That invention, in turn, was an improvement over U.S. Pat. No. 3,304,925, also invented by the instant applicant's father.
According to the first patent referenced above, a small flat was ground on the outer surface of the plunger that is internal of the lifter body. This flat had the effect of communicating continuously between the high pressure chamber in the bottom of the lifter which becomes pressurized and raises up to lift the valve pushrod higher, which opens the valve longer. The flat acts as a bleed passageway. The cross sectional area of the passageway defined between the flat and the cylindrical lifter body in which the plunger slides is very restricted. Because of the exponential increase of the resistance to fluids that an orifice offers at increased flow rates, the passageway adequately bleeds the lifter pressure chamber at low RPM, but is substantially ineffective at high RPM.
This has the effect of reducing valve duration significantly at low RPM while permitting normal hydraulic pumping up of the lifter at high RPM to achieve the enlarged lift and duration that the modified camshaft was installed to provide.
The flat that was ground on the lifter plunger according to the above-referenced patent disclosure did not extend the full length of the plunger. It only extended from the bottom end in communication with the lifter pressure chamber to an annular channel midway up the plunger required for oil circulation. This channel communicates with oil under pressure as provided by the oil pump, and thus it would seem logical that to grind the flat the remaining distance, all the way to the top of the plunger, would have a detrimental effect on oil pressure. A flat so ground would directly communicate between the pressurized oil in an oil gallery, and the sump area of the engine which is substantially at atmospheric pressure.
Because the flat could not be ground all the way up each plunger, each plunger had to be individually ground. The operator would have to be very careful to stop the grinder, which ordinarily would be an abrasive wheel on a surface grinder, from passing beyond the annular oil port in the plunger. Because of this, the labor intensity of reducing the modified plungers was considerable. The instant inventor has been making and selling valve lifters according to the above-referenced patent for ten years. Each and every valve lifter produced during that ten-year period was individually ground, with the flat stopping at the annular channel. During this time, it did not occur to the inventor that he could grind the flat all the way up without losing oil pressure. This is quite significant, because the inventor makes his living selling these lifters with the flat ground on the side of the plunger. He is an expert on modified lifters, and for ten years it was not obvious to him to change his manufacturing process by grinding the flat across the entire length of the plunger.
Before making the new plunger with the full-length slot or flat, the inventor's dubious opinion was corroborated by mechanics and engineers who also felt that it was essential to have a positive seal between the oil gallery and the engine's sump around the top portion of the lifter plunger. However, the inventor's sales had grown over the years to the point where the labor involved in producing the plungers the old way became a burden. In order to be able to carry on, the inventor tried continuing the passageway over the entire side of the lifter. To everyone's tremendous surprise, the lifters worked just as well with the passageway all the way up as the passageway stopping at midsection of the lifter.
Although structurally what the inventor achieved might not appear significant to one not involved in the business on a day-to-day basis, the effect of the new-style passageway on the inventor's business operation is tremendous. Rather than being required to individually grind each plunger, an entire set of sixteen is coaxially aligned on the magnetic chuck of a surface grinder, and the grinding wheel modifies the entire set at once. He can produce hundreds of modified lifters in the time previously required to produce dozens.
And, just as important, the new production technique does not cause a deterioration of lifter operation or loss of oil pressure at all. This is very important because the Rhoads lifter, as it is known in the marketplace, has achieved a reputation for quality that has been developed over years and this good will could not be jeopardized, even though substantial labor might be saved by producing a lifter of decreased performance.